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Electrochemistry Communications 7 (2005) 995–999

Large-area network of polyaniline nanowires preparedby potentiostatic deposition process

Vinay Gupta a,b,*, Norio Miura a

a Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japanb Japan Science and Technology Agency, Kawaguchi-shi, Saitama 332-0012, Japan

Received 25 May 2005; received in revised form 30 June 2005; accepted 4 July 2005Available online 10 August 2005

Abstract

Polyaniline nanowires (mean diameter 30–60 nm) have been deposited directly on a stainless steel electrode by the potentiostaticprocess. This procedure resulted in the large-area (1 · 1 cm) three-dimensional network of polyaniline nanowires. The nanowire�smorphology can be controlled by means of appropriate deposition condition. A possible growth process of the formation of thepolyaniline nanowire network is proposed.� 2005 Elsevier B.V. All rights reserved.

Keywords: Polyaniline; Nanowire; Scanning electron microscopy

1. Introduction

Polyaniline has a wide range of interesting electrical,electrochemical and optical characteristics along withexcellent environmental stability [1–4], which makespolyaniline useful in a range of applications such aschemical sensors [5,6], gas separation membranes [7],supercapacitors [8], display devices [9] and anticorrosioncoating [10]. Recent synthesis of polyaniline in the nano-material-form has increased its significance multifolddue to its superior characteristics at nanoscale [11,12]in comparison to macroscopic-size polyaniline.

Up to now, nano structured polyaniline, with differ-ent morphologies, has been synthesized using varioustechniques such as template synthesis [13], self-assembly[14], emulsions [15] and interfacial polymerization [16].Polyaniline spheres and needles have been synthesizedby a solid-stabilized emulsion [17]. Hexagonal plates ofpolyaniline have been synthesized by ultrasonic irradia-tion [18]. Polyaniline nanowires and nanotubes have

1388-2481/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2005.07.008

* Corresponding author. Tel./fax: +81 92 583 7886.E-mail address: drvinaygupta@netscape.net (V. Gupta).

been synthesized using a solid template, such as porousmembrane [19]. Oriented nanowires of polyaniline havebeen fabricated by combining self-assembly and tem-plate synthesis technique [20]. A high yield polyanilinenanowire has been obtained using a soft template [21].Among all such techniques, emulsion technique givesrather higher yield in comparison to other methods.However, it requires relatively large amount of surfac-tants serving as emulsifiers and it is rather tedious to re-cycle the surfactants after polymerization.

Most of the methods that have been used to preparepolyaniline nanostructures are indirect and need sophis-ticated techniques to apply the nano structured polyan-iline onto a substrate for practical applications. It wouldbe great technological importance to be able to depositpolyaniline nano-structure directly on the desired sub-strate for a particular application in a simple and costeffective way. However, the direct deposition of polyan-iline nanowires with different morphologies and over alarge-area of the substrate with high yield was not re-ported so far. In this work, the synthesis of polyanilinenanowires has been achieved potentiostatically at theconstant potential of 0.75 V on the stainless steel over

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a large area of 1 · 1 cm. This is the largest polyaniline-nanowires-network ever synthesized. A growth mecha-nism of the polyaniline nanowire is proposed.

2. Experimental

The aniline and the 1 M H2SO4 were obtained fromWako� Chemicals. Research grade stainless-steel (SS)(grade 304, 0.2 mm thick) was obtained from the Nila-co� Corporation. The SS was polished with emery paperto a rough finish, washed free of emery particles andthen air-dried. An electrochemical-cell was assembledin three-electrode configuration in which the counterelectrode was platinum (Pt), the reference electrodewas saturated calomel electrode (SCE) and workingelectrode was SS. The total deposition area of the SSwas 1 · 1 cm and the separation between Pt and SSwas 1 cm. The electrochemical deposition was per-formed using auto-lab� PGSTAT 30 instrument (Ecochemie, Netherlands, http://www.ecochemie.nl) con-nected to a three-electrode cell. An electrolyte solutionof 1 M H2SO4 + 0.05 M aniline was used for the electro-chemical deposition of polyaniline nanowires on SS elec-trode. The deposition of polyaniline was carried out atthe constant potential of 0.75 V for several minutes.Subsequent to deposition, the electrode was washed indistilled water and dried in oven at 40 �C for a day.The microstructure and the thickness of the compositeswere evaluated by means of JEOL scanning electronmicroscope (FE-SEM, JEOL, JSM-6340F). Polyanilinenanowires were also characterized for UV/visspectroscopy.

Fig. 1. (a)–(c) SEM images of the polyaniline nanowires in theincreasing order of magnification.

3. Results and discussion

The large-area, three-dimensional network of polyan-iline nanowires, formed in 12 min deposition time, isshown in Fig. 1. The SEM micrographs of polyanilinenanowires are shown in increasing magnification fromFig. 1(a)–(c). The nanowires surface is very smooth withhomogenous diameter. From the SEM images, the aver-age diameter is estimated to be 30–60 nm. The thicknessof the deposit network is �20 lm, area and mass is0.26 mg. The morphology of the nanowires is highlyhomogenous and it shows that electrochemical processis effective in the deposition of polyaniline nanowiresuniformly over a large area.

An increase in the potential of deposition to 0.8 V re-sulted in very low content of nanowires, which may bedue to the conversion of polyaniline from emeraldine(EM) to pernigraniline (PE) form at this potential [1–4]. Lowering the potential below 0.7 V resulted intosharp decrease in the current (�1 mA) to half value,which also resulted in low content of nanowires. Other

methods to prepare such nanowires, such as potentiody-namic method, were not successful because in suchmethods, oxidation and reduction occurs simulta-neously. Even in the indirect synthesis of polyanilinenanowires, only oxidative polymerization methods weresuccessful [5–21].

It was expected that with increasing deposition timefor further few minutes the thickness of polyanilinenanowire deposit would increase. Instead, it is found

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that the surface of the wires surface changes to corn-like morphology without significant change in thethickness, as shown in Fig. 2(a). The diameter of thenanowire is reached closed to 100 nm but the thick-ness was increased by only several micrometers inthe corn-like morphology of polyaniline nanowires.A further deposition for few minutes resulted in fillingof the porous network of polyaniline nanowires asshown in Fig. 2(b). At this point of the deposition,the increase in the mass is almost twice withoutdetectable change in the thickness.

The sequence of morphological changes in the poly-aniline nanowire network is more clearly shown inFig. 3 at lower magnification. As shown in Fig. 3(a),at the beginning of the deposition, the nanowires arealigned more approximately perpendicular than parallelto the SS electrode. As the deposition progresses, theyacquire corn-like morphology due to the deposition ofthe polyaniline on the surface of the nanowires, asshown in Fig. 3(b). Finally, all the porous area of the

Fig. 2. (a) SEM image of the corn-like polyaniline nanowires. (b) SEMimage of the polyaniline nanowires with polyaniline particles in acomposite form.

Fig. 3. SEM images of the changes in the polyaniline nanowiresnetwork morphology in the order of increasing polyaniline content:(a) 0.26 mg; (b) 0.37 mg; (c) 0.59 mg.

network is filled with polyaniline nano particles suchthat the nanowire�s are no longer visible, as shown inFig. 3(c).

Based upon these observations, a possible growthprocess of such nanowires is proposed as shown inFig. 4(a)–(d). The polyaniline nanowires are expectedto grow via seedling growth process, in which furtherpolyaniline is deposited on the initially deposited nano-

Fig. 6. Complex impedance spectra of polyaniline nanowires atdifferent potentials.

Fig. 4. (a)–(d) Schematics of the possible growth process of thepolyaniline nanowires in the order of increasing polyaniline content.

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sized granules (Fig. 4(a)). As the deposition progress, analigned nanowire network is formed (Fig. 5(b)). A fur-ther deposition results in extended length and in the mis-alignment of nanowires and cross-links are formed.From there, the secondary growth of polyaniline beginsas the incoming polyaniline is distributed along the elon-gated surface of the nanowires which results in theircorn-like structure as shown in Fig. 4(c). Polyanilinecontinue to deposit on the network of the nanowiressuch that all porous area is filled, as shown inFig. 4(d). The final situation is similar to as shown inFig. 3(c).

Fig. 5 shows the UV/vis absorbance spectra of thepolyaniline nanowires in doped state. The absorbancespectra maxima close to 830 and 390 nm, in close agree-ment with previously reported nanosized emeraldine indoped state [22]. Fig. 6 shows the complex impedancespectra of the polyaniline nanaowires at different ap-plied potentials. The initial non-zero intercept at Z 0 atthe beginning of the semicircle is almost identical inall the curves and is due to the electrical-resistance(RX) of the electrolyte, which have the average valueof 0.5 X in 1 M H2SO4 electrolyte. The resistance pro-jected by the semicircle is due to the resistance of theactive electrode material (Re). Therefore, the resistance

Fig. 5. UV/vis spectra of polyaniline nanowires in doped emeraldinestate.

values of the polyaniline nanowires is in the range of0.4–0.6 X. This implies that polyaniline nanowires arehighly conducting and can be used over a wide rangeof potentials. A comparison of the impedance spectraof the polyaniline nanostructures, given in Fig. 3, isshown in Fig. 7. An increase in the resistance occurswhen polyaniline nanowire is changed to corn-like mor-phology (Figs. 7(b) and 3(b)). For a non-nanowiresmorphology (Fig. 3(c)), the resistance increase is serentimes as compared to nanowire, as shown in Fig. 7(c).

Fig. 7. Complex impedance spectra (at 0.45 V) of polyaniline nano-wires network in order of increasing polyaniline content. (a) 0.26,(b) 0.37 and (c) 0.59 mg, corresponding to the SEM images in Fig. 3.

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4. Conclusion

Highly conducting polyaniline nanowires have beensynthesized over a large-area (1 · 1 cm) and in high yieldusing a very simple electrochemical technique. The for-mation of such nanowires can be explained invokingseedling growth process. Polyaniline nanowires of differ-ent morphologies can be synthesized using appropriateexperimental conditions. Such nanowires can have manyapplications such as in sensors and energy storage.

Acknowledgement

This present work was supported by Japan Scienceand Technology (JST) agency through ‘‘Core researchfor Evolution Science and Technology (CREST)’’ underthe project ‘‘Development of advanced nanostructuredmaterials for energy conversion and storage’’.

References

[1] A.G. MacDiarmid, J.-C. Chiang, A.F. Richter, N.L.D. Somasiri,A.J. Epstein, in: L. Alacer (Ed.), Conducting Polymers, D. ReidelPublication, Dordrecht, 1987, p. 105.

[2] J.C. Chiang, A.G. MacDiarmid, Synthetic Met. 13 (1986) 193.

[3] A.G. MacDiarmid, Synth. Met. 84 (1997) 27.[4] A.G. MacDiarmid, Angew. Chem. Int. Ed. 40 (2001) 2581.[5] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, J. Am. Chem. Soc.

125 (2003) 314.[6] H. Liu, J. Kameoka, D.A. Czaplewski, H.G. Craighead, Nano

Lett. 4 (2004) 671.[7] J. Yang, S.M. Burkinshaw, J. Zhou, A.P. Monkman, P.J. Brown,

Adv. Mater. 15 (2003) 1081.[8] K.R. Prasad, N. Munichandraiah, J. Power Sources 4944 (2002) 1.[9] L. Huang, J. Liu, C.F. Windisch, G.J. Exarhos, Y. Lin, Angew.

Chem. Int. Ed. 41 (2004) 3665.[10] A.G. Macdiarmid, Synth. Met. 84 (1997) 27.[11] L. Liang, J. Liu, C.F. Windisch, G.J. Exarhos, Angew. Chem. Int.

Ed. 41 (2002) 3665.[12] Y. Ma, J. Zhang, G. Zhang, H. He, J. Am. Chem. Soc. 126 (2004)

7097.[13] Z. Niu, Z. Yang, Z. Hu, Y. Lu, C.C. Han, Adv. Funct. Mater. 13

(2003) 925.[14] H. Qiu, M. Wan, B. Matthews, L. Dai, Macromolecules 34 (2001)

675.[15] Z. Wei, M. Wan, Adv. Mater. 14 (2002) 1314.[16] J. Huang, R.B. Kaner, J. Am. Chem. Soc. 126 (2004) 851.[17] Y. He, Mater. Lett. 59 (2005) 2133.[18] H. Liu, X.B. Hu, J.Y. Wang, R.I. Boughton, Macromolecules 35

(2002) 9414.[19] C.G. Wu, T. Bein, Science 264 (1994) 1757.[20] H. Qui, J. Zhai, S. Li, L. Jiang, M. Wan, Adv. Funct. Mater. 13

(2003) 925.[21] W. Zhong, J. Deng, Y. Yang, W. Yang, Macromol Rapid

Commun. 26 (2005) 395.[22] D.M. Sarno, S.J. Manohar, A.G. MacDiarmid, Synth. Met. 148

(2005) 237.